Many of conventionally mass-produced piezoelectric ceramics are formed of PZT (lead zirconate titanate) materials and thus contain lead. However, in recent years, in order to eliminate an adverse effect of lead on the environment, development of lead-free piezoelectric ceramics has been desired. A composition expressed by compositional formula ANbO3 (A indicates an alkali metal), such as potassium sodium niobate ((K, Na)NbO3), is proposed as material for such a lead-free piezoelectric ceramic (called “lead-free piezoelectric ceramic composition”). However, an ANbO3 lead-free piezoelectric ceramic composition involves a problem in that sinterability and resistance to humidity are poor.
In order to cope with such a problem, Patent Document 1 mentioned below discloses a method of improving sinterability and, in turn, piezoelectric constant d33 and electromechanical coupling factor kr by adding Cu, Li, Ta, or the like to an ANbO3 lead-free piezoelectric ceramic composition.
Also, Patent Document 2 discloses a lead-free piezoelectric ceramic composition expressed by general formula {Lix(K1−yNay)1−x}(Nb1−zSbz)O3 (0≤x≤0.2, 0≤y≤1.0, 0≤z≤0.2 except x=z=0) which exhibits relatively good sinterability and piezoelectric properties (piezoelectric constant d33 and electromechanical coupling factor kr).